Ceramic materials have a variety of industrial and scientific uses, the most common of which is use in separation processes. Ceramic materials also find utility as adsorbents and catalysts, and need not be provided in membrane form. Organic membranes are often currently used in industry for separation processes, but ceramic membranes offer several advantages over organic membranes. Ceramic materials are more resistant than organic materials to organic solvents, chlorine, and extremes of pH. Ceramic materials are also inherently more stable at high temperatures, thus allowing more efficient sterilization of process equipment than is possible with organic materials. Ceramic materials are generally quite resistant to microbial or biological degradation, which can occasionally be a problem with organic materials. Ceramic materials are also more mechanically stable under high pressures.
The mechanism of operation and types of separations which can be achieved by ceramic membranes are discussed in general by Asaeda et al., Jour. of Chem. Eng. of Japan, 19[1]: 72-77 (1986). At least one line of ceramic filters is currently marketed under the trade name "Ceraflo" by the Norton Company of Worcester, Mass.
Ceramic materials may be formed in particulate or polymeric manners. Anderson, et al., J. Memb. Sci 39: 243-258 (1988), describes different methods of making both particulate and polymeric sols from transition metal oxides. In general, particulate materials have a smaller average pore diameter and a narrower pore size distribution as compared to polymeric materials.
Particulate ceramic materials are typically formed through a process beginning with metal-organic compounds. The compounds are hydrolyzed to form small metal oxide clusters which in turn condense or aggregate to form metal oxide particles. The particles are fused into a unitary ceramic material. The gaps between the fused particles form a series of pores in the material.
The creation of these metal oxide ceramic materials is generally conducted through a sol-gel procedure. Usually, the metal oxide is initiated into the process as a metal alkoxide solution. The metal is hydrolyzed to metal hydroxide monomers, clusters or particles, depending on the quantity of solvent used. The insoluble metal oxide particles are then peptized by the addition of an acid, or in some cases a base, which causes the particles of the metal oxide to have a greater tendency to remain in suspension, presumably due to charges acquired by the particles during the peptizing process.
Such a sol can be evaporated to form a gel, which is a semi-solid material. Further evaporation to a xerogel, and then sintering of the xerogel results in a durable rigid material which can either be formed as an unsupported material or as a supported material coated onto a substrate. This substrate can be either porous or non-porous and either metallic or non-metallic, depending on the particular application.
Two current limitations on the use of ceramic materials are the fabrication materials and the material pore size and distribution range. With regard to the composition of the materials, ceramic materials have been created using many starting materials. For example, Leenaars et al., Jour. of Membrane Science, 24: 261-270 (1985), report the use of the sol-gel procedure to prepare supported and unsupported alumina membranes. However, this particular sol-gel process had inherent lower limits as to the size of the particles that could be produced in the sol thus making a lower limit on the size of the pores in the membranes. Ceramic membranes composed of titania, zirconium and other metal oxides have also been reported.
Alumina membranes have been extensively studied. In particular, Yoldas conducted significant research on the fabrication of gamma-alumina membranes made by a sol-gel process. Yoldas was able to achieve a relatively small particle size in the sols and was able to achieve porous membranes. Yoldas, Jour. Mat. Sci., 12:6, pp. 1203-1208 (1977). In general, however, the method of Yoldas did not permit sufficiently fine control over the sol-gel process to produce uniformly small particles. As a result, the resulting membranes proved less useful for filtration, catalysis or adsorption. For example, in the transparent alumina membranes Yoldas reported that he was able to achieve materials that had small pores, having a radius less than 50 Angstrom, but only in conjunction with the materials which had "larger river-like pores" having a significantly higher radius and contributing significantly to the porosity of the material. Yoldas, Cer. Bull., 54:3, 286-288 (1975).
Yoldas also investigated the manufacture, through the sol-gel processes, of mixed alumina and silica materials. One class of materials which Yoldas prepared were alumina-siloxane derivatives which formed polymeric cross-linkages making polymeric, rather than particulate, ceramic materials. Yoldas was also able to make several large dense monolithic glass samples of varying compositions of silica and alumina that did retain some porosity and high surface area, as described in the article in Jour. Mat. Sci., supra. Yoldas did not report any ability to make particulate aluminosilicate porous membranes, or the ability to make aluminosilicate membranes having high porosity with exceedingly small pore sizes approaching those of the alumina membranes which he had made.
Substituted silicates, such as aluminosilicates having Si as the major component, exhibit a number of advantages such as thermal and chemical stability, mechanical strength and active surface properties. These materials have wide application in the areas of catalysis, catalyst support and separation. Most commercially available porous aluminosilicates are varied zeolites. Recently, the use of amorphous aluminosilicates as acid catalysts has attracted great attention. One of the reasons is that compared with zeolites, the synthesis of amorphous silicates can be carried out under more moderate conditions, for instance, room temperature and pressure.
In certain application areas, such as catalysis and separation, microporous ceramic materials are desired. In these cases, the products must have a large pore volume, a high surface area and a pore diameter as small as 5-10 .ANG.. At the same time, these ceramic materials must have a stable microstructure to withstand sintering at high temperature and under harsh chemical conditions.
Sol-gel methods have been employed to fabricate aluminosilicates in several published studies. Most previous authors have applied sol-gel techniques to prepare various aluminosilicates either as dense monoliths or as powders. When alkoxides are used as precursors, a prehydrolyzed tetraethyl orthosilicate (TEOS) is usually mixed with an aluminum alkoxide followed by hydrolysis and condensation of this molecular level mixture. However, Al--OR and Si--OR bonding have different hydrolysis rates which can cause non-homogenous products. Nanoscale mixed gels of diphasic alumina and silica were obtained by mixing boemite and silica sols, both having colloidal particles in the nanometer size range. Thermal stable mesoporosity was found in these gels. These mixed gels have two phases even after being fired at 1000.degree. C.
In order for the materials to be useful for filtration, the pore size of the material is preferably within a relatively narrow range, so that larger species are excluded from the material passing through the filter. It is also useful to achieve pore sizes of less than 100 Angstroms, which are useful for many separation applications. Examples of such applications include ultrafiltration, reverse osmosis, molecular sieving and gas separation. The ability to achieve materials having a defined pore size which is even less than 20 Angstroms has significant additional advantage.
In one earlier publication, Anderson and Chu have reported a sol-gel method for preparing pure silica gels having high surface area and pore size as small as 10 .ANG.. Unfortunately, the utility of these gels is limited by poor thermal and chemical stability under some conditions.
Anderson and Sheng have shown that aluminosilicate ceramic materials can be made by mixing together and heating an alumina sol and a silica sol or from a single mixed alumina/silica sol. By varying the relative concentrations of alumina and silica in those materials, one varies the charge properties of the materials formed. Thus the charge properties of the mixed ceramic materials of Anderson and Sheng are intermediate between those formed of pure silica and those of pure alumina.
It would be desirable to produce ceramic materials having microstructural properties similar to pure silica ceramic materials, yet having improved thermal and chemical properties.